Climate has always changed, why are we worried now and why does it have to be humans fault?

Answer:
Yes, climate has varied in the past and it has varied for many different reasons, some better understood than others. The present day climate change is very well understood and is different. Simply noting that something happened before without humans does not in any logical way show that humans are not causing it today.

For example, we see in ice core records from Antarctica and Greenland that the world cycled in and out of glacial periods over 120Kyr cycles. The cause for that climate cycle’s timing is fairly well understood to be the results of changes in the orbit of the Earth, though the mechanism behind the resulting response has not been conclusively established. These orbital cycles are regular and predictable and they are definitely not the cause of today’s warming. The other important difference between the glacial-interglacial cycles and today is the rapidity of the current change. The rate of warming is on the order of 10 times faster today than seen in the ice cores.

Such rapid warming on a global scale is very rare in the geological record, and while it may not be unprecedented, there is very strong evidence that whenever such a change has happened, whatever the cause, it was a catastrophic event for the biosphere.

“Climate is Always Changing” was first published here, where you can still find the original comment thread. This updated version is also posted on the Grist website, where additional comments can be found, though the author, Coby Beck, does not monitor or respond there.

Comments

Oh happy day!
Here’s a link from Australia’s ABC News website. (which I read every day)
Breathe easy everyone! We have fixed the hole in the ozone layer! It’s taken 22 years since the Montreal Protocol, but thankfully, the banning of CFCs has caused it to get smaller! Now it’s only the size of Antarctica! (tiny really)

Watch out though! It seems there’s another evil gas that’s sure to kill us all. A link on the same page says that Nitrous Oxide may have an even worse effect on the ozone layer!
Humanity, take action!
We can fix all these problems. This story proves it!

But seriously folks, this is an article that is a great example of the AGW movement’s propaganda.
It’s an article that basically says nothing either way, but “introduces” or emphasises” the idea that human
Action(!) can fix the earth’s troubles. We have a lot to fear, but we CAN save the world by implementing such “measures” as Emmissions Trading Schemes.

Let’s not forget, if I can state the bleedin’ obvious that we are here to debate ANTHROPOGENIC climate change/warming.
Not one of us filthy deniers, here or anywhere, are denying climate change.
NOT ONE!! Nor, are we denying that the earth can warm or cool.
That is part of the reason why I posted a fair while ago, that this blog should be titled correctly instead of misleadingly, insinuating that a “denier” is too stupid to read warming trend graphs, and satelite data.
Coby, I know you’re busy with kids ‘n stuff, but I’d appreciate (being a stickler) you changing the title of this blog to “How To Talk To a Human Induced Climate Change Skeptic” or even “How To Talk To An AGW Denier”.
We deniers (speaking for myself mostly) are not “evil capitalists” looking for a short term dollar at the expense of the entire earth.
We are simply people who don’t believe the propaganda fed to us by the mainstream media, and want to know the real story of something.
I was a believer after I first saw Al Gore’s movie, but then I started to question his “facts” and the more I questioned, the more I learnt.
That’s the crux of it.

As usual, I await the usual cherry-picked abusive response from those opposed.

michael? you say all you want to know is “the real story” ? seriously? ok. here ya go: we live inside a bubble suspended in a vaccuum and we are pumping the bubble full of poison gases from the inside… and you want to defend your “right” to do so? why? if not for a “short term dollar at the expense of the entire earth”, then.. why?
though you may think that you are valiantly battling the evil “propaganda” giant the truth is all you are really doing is defending your “right” to pollute the air. AGW advocates call for actions to be taken to reduce emissions. why is that such an evil thing? why do you insist on attacking is so fervently? please respond. i must know why you bother to argue such a degenerative view. i rarely pipe up on this type of thing but your above comments are so ludicrous that i was truly inspired…

Hi Solar,
When I say the “real” story, I mean that I like to get as much information on a given topic as I can find, and try to work it out for myself.
I consider myself to be an environmentalist.
I have posted here previously that I strongly believe in reducing and limiting pollution.
If you have been to somewhere like Hong Kong and seen the air so thick with Anthropogenic Pollution that one may look directly at the sun at two in the afternoon, then you understand where I’m coming from.
I live in Brisbane, Australia and regularly travel to the World Heritage Listed, Fraser Island.
It it a “pristine” sand island which is littered with plastic bottles, bags, cans and various other waste!
From the reading I’ve done so far, I simply cannot agree that one element, namely Carbon, and the human release of it, is alone responsible for the apparent warming, or for that matter Climate Change in general.
I have been involved in the Solar Electricity industry here in Australia, and for a country with more sunlight that most others, our governments simply haven’t embraced it at all.
At least the AGW movement has increased research funding in Photovoltaics!
The reason I think it’s so urgent and important to get the Climate Change debate right is that Australia’s Federal Government is trying to introduce an emissions trading scheme that will have no influence whatsoever on the climate of the earth, and begin to destroy the industry and jobs of Australia and it’s neighbours.
When all this first became a big issue, after the release of An Inconvenient Truth, I was a believer and would tell anyone who would listen that we are in trouble, and they should see that movie.
I read as much as I can of both sides of the debate and so far, I have been convinced by the “skeptical” rather than the “belevers”.
I hope I am making sense. I’m writing this in between taking business phone calls and replying to emails.

Usually, I have found that posts such as mine are responded to with abuse and “cherry-picked” criticism.
I guess we shall see.
I’d like to know what you think.

Michael, we have seen one side of the story here supported by evidences. Can you share with us what is the other side of the story that you seen and convinced you to believe in the opposite of the theory that best fit these evidence?

Ahh, but that’s just the trouble wlark.
Even before (actually, long before) the whole “email” thing surfaced, (I refuse to use the word “gate” after any other word to refer to a controversial event!) I did not believe the IPCC was a scientific organisation.
I do not believe that the “bodies” wot produce the
so-called “evidences” are bodies that are to be believed or trusted!
There seems to me to be a rather vocal group of scientists (climate or otherwise) that want their opinion heard, only to have it fall on deaf ears….http://www.oism.org/pproject/
I have written here ad nausaeum on my environmental position, and that I believe wholeheartedly that the IPCC is a political organisisation and not a scientific one.
I was worried and alarmed by Mr Gore’s movie when I first saw it. But I try to read as much as I can, and I have decided that I believe the other side.
(i.e. the side that says climate change is very real, but is probably not caused by human activity)
I’ll say again… I consider myself an environmenatalist. I believe pollution is bad and must be reduced. (human activity causes pollution)
I believe the climate is changing, and we humans can adapt or perish. The machinations of Mother Earth are not considerate of Carbon Credits, Emissions Trading Schemes, or COP15!
I also believe that global warming will be good for life on earth. I believe that global cooling is bad for life on earth. Global cooling causes deserts to expand. Global warming causes deserts to shrink. A read of human history proves it. In warm times, life flourished. In cold times, life suffered.
The earth will never have a runaway greenhouse effect like Venus. (there is a slight difference in distance from the sun god RA!)
I’m sorry, but I don’t have links to my assertations. I would like, however, to be “on the record”.
As usual, I await the usual abusive replies….

Michael, scientists use empirical evidence to deduce facts, use logic to explain the facts and use statistics to show that they are correct. Deniers, like you, “believe” anything which supports their selfish and greedy personae.

Michael.
I also live in Australia (Adelaide), so I share some of your views about our government’s actions. And I am also an environmental scientist (wildlife) and work for the government (public service).
However, taking some of your earlier points. Firstly, there are three types of deniers. Despite your view that they don’t exist, there is the flat earth society, who consistently refuses to accept that the climate is changing at all. You can read their writings all over the place, even here. There is no point talking to them, because their views are – well – stupid.
The second type of denier are those who accept that the climate is changing, but deny that the causes are anthropogenic. The problem is that a lot of these people are simply some of the more pragmatic of the flat earth society who have realised how untenable it is to deny any form of climate change, and who have now moved on to the second stage of denial. Once again, there are a lot of this type around.
The final ‘denier’ are those (very small in number) who accept climate change is real, but who are yet to be convinced about the ‘proof’ that the causes are anthropogenic. The problem with this view is that the ‘proof’ is out there in virtually unlimited amounts. You are quite correct to be sceptical about the mainstream media’s ability to convey this information adequately, so my suggestion is to ignore it. How about you go to the source? Read some of the papers which outline the evidence for anthropogenic climate change. The IPCC is a political organisation, not a scientific one, and the IPCC reports are not source documents. But there are hundreds of links in the IPCC reports to source documents. Its a good place to start. DO NOT READ BLOG POSTS – they are the worst place to gather evidence or to come to a viewpoint. The ability to write pursuasively does not make a case more valid – and unfortunately, a lot of scientists make appalling authors.
So go ahead – be sceptical. But be a true sceptic, not a denier. Read the information with a critical eye – and if you do that with an open mind (not a closed one that starts from the position of denial), then I am certain there will be only one outcome.

Michael.
And I would caution you to be very careful in your assertions about the ability to adapt and warming being good / cooling being bad. That is not the case.
Rapid climate change has been the cause of mass extinctions in the past, and that would likely be the case again. It would also have a huge impact on things like agriculture, as the favourable zones for crops shifted. Changes in sea level and ocean temperature will also have huge impacts on coral reefs etc – with potential places like the GBR to simply cease to exist. Imagine how that would affect the Queensland economy, let alone the ecosystem.
So think a little harder before you come to a conclusion. Climate change will be a disaster in the short term – no matter what happens.

Aw poor little Ian, he is just one more poke away from calling me all names his limited vocabulary can hold.

Seriously Ian I have looked at your 4 reports and as yet i cannot find the answer to my question. I was hoping you could help? Can you be a bit more specific? Maybe you could start with which of the 4 reports might show the empirical evidence of water vapour/clouds acting as a positive feed back?

You are no different to Ian, you might preach a good sermon but it is non existent in fact. I have read all the things that you spoke of and most of it fails on the one critical point, empirical evidence that proves the theory. I am now waiting for Ian to dodge my question just like you have dodged it in the past, if you are so sure that i fit into one of your 3 little pidgeon holes then you should have no problems in helping Ian out and at the same time converting myself and Michael, so how about it Mandas?

I had a read mind you i did not take long as it was only one page long! One page to discuss the fundemental element of a theory it sounds a bit light to me.

This link/page simply discusses the computer models used by various people for example:

Cess et al have a nodel of absorption of solar radiation by clouds which by the way was not accurate.

Hall & Manabe (another computer model) they make the assumption WV is positive but how much no one knows they then go on to state that clouds magnitude and sign are unknown. In other words they are just guessing. Held & Soden were much the say.

So Ian surely you do not rely on this one page from an IPCC report which simply lists inadequate computer models as your evidence, remember i asked for empirical evidence that shows WV/clouds will act as a positive feedback causing the catastrophic climate change you hope and belive will happen.

Surely i have the justification to ask what evidence you base these assertions on, the link you supplied falls well short of what would be considered scientific evidence, even a scientist like yourself must surely agree with me on this.

Ian Robert Forrester!! (a guess)
You go straight to your room young man!
You continue to attempt to abuse and insult. For God’s sake, WHY!???
As Craker so rightly asks, haven’t you got anything other than IPCC links? Is the IPCC and their endorsed and vetted sub-sources your only source? (is this a case of horses for sources? OR, horses for sauces?! OR, horses for causes??!! Bahahaha!!!!)

Mandas, I thank you for your reasoned and non-abusive replies.
(I think I fit in to your third category by the way…)
As I have written here before, I do believe the Earth has a climate. I also believe the climate changes.
I do not believe the Proof is Out There in abundance.
Here are two links to the same person.
(one an interview on Brisbane radio, and the other, her site. I hope the audio link works.)

http://joannenova.com.au/
I hope you will listen and read. (your mind may be open or closed)
I stand by what I’ve written in my posts.
Now, regarding the imminent inundation of various pacific islands… I am in Queensland and you are in South Australia, but isn’t the ocean water between us all connected?
Are the sea levels rising in South Australia?
If The Maldives, and Fiji are being inundated, surely we would see a sea level rise too, wouldn’t we?
Is the global use of the term “sea level” dependent on which country you’re in? I mean, Denver, Colorado is around 1 mile above sea level, and Mount Kosciusko in Australia is 2300metres above sea level (yes, I looked it up). Do we in Australia need a formula to convert from/to American Sea Level?
(let’s see…. there’re ten chains in a furlong, and 14 pounds in a stone….a US Gallon is less than and Imperial Gallon)
How can we be sure that The Rocky Mountains are actually higher than ours?
Are the Greek Islands experiencing or expecting catastrophic sea level rise too?
Is my point clear?
I think so.
What do you think of the “publicity” surrounding the COP15 summit? What did you think of the opening video? What do you think of the various cabinet meetings in scuba gear?
I’d like to know what you think. (and you Skip, and you Coby)

Crakar, one page wasn’t enough? OK, Here’s 11 pages for you to find (if you can’t find it on the internet I’m sure you know where your local library is). Of course these 11 pages don’t cover the whole topic, but get back to me when you’ve read them & I’ll give you some more (ISI WoS lists 106 papers in Meteorology & atmospheric sciences using the search term “water vapour feedback” so there’s plenty more reading to come for you)

Here’s a Youtube link to a six part program that I thought was very interesting indeed. It’s called The Cloud Mystery.
(I mean the scientific message of it. I didn’t like the “dramatisations” of day-to-day interactions between colleagues as they do in current affairs shows)
I think it’s especially interesting to hear what they say about the reaction to their research by the IPCC.

Regarding my previous post, (#20) I don’t mean to sound glib by writing what I did about sea levels.
I would genuinely like to know what everyone thinks of that. (without name-calling please?) As I wrote previously, the “underwater cabinet meetings” and the COP15 opening video are great examples of fear mongering, that are not based on any scientific fact at all.
(am I wrong about this?? Are the movies 2012 and The Day After Tomorrow, actually documentaries?)

I would really appreciate your opinions.
But may I say, Ian Robert Patrick Forrester! If you write anything at all that contains the words, stupid, brainless, idiotic, or any other childish insult, you WILL be straight back to your room! Is that clear?

Michael, I’m no authority in climatic change but in response to your question regarding uniformity of the degrees of sea level rises. Depending on where in the world you make your observations, sea level changes (or indeed, upward movement of a landmass) may occur over time from isostatic adjustment. This is when an area of the earth previously covered in ice, ‘bounce’ post glacial retreat, in respose to the load weight being reduced by melt.

Taking the UK as an example of a island off the European continent, this explains how the North Sea (English Channel) formed. Another example would be the existence of the Isle of Wight, separated from the UK mainland by the stretch of water known as the Solent. Evidence exists of (now)submarine forest and soil samples from below the sea bed. The soil samples reflect in the lineal similarities exhibited on dry land.

The last paragraph in the OP seems to switch topics. You seem to admit that sudden climate changes such as what we see now are not unprecedented. You then try to negate this argument by saying that historically, such changes always caused catastrophic events for life on the planet. I can agree with that, but that seems to be beside the point. We need to establish first that the current climate change event is anthropogenic. If it’s not, then sure, you can say it will be catastrophic, but there isn’t much we can do about.

So, if there have been other warming episodes in the past, is there some evidence to suggest that the current warming cannot be a repetition of any of them? In other words, how do we know that whatever triggered some episode several million years ago is not triggering today’s warming? Sure, there are triggers that we can rule out. For example, you say current warming is not caused by milankovich cycles. In other articles, you say some other episodes (such as P-E thermal maximum) have known causes and are not similar to what’s happening today. But how many others can we actually rule out? I’m looking for something like this. Take last T episodes of global warming. M of them are due to milankovich cycles, so we can rule them out. Of the remaining, we rule out K more, because they have some known cause, or, say, the rate of warming is different from what we see today. And we don’t have enough data to rule out any others. So we have U = T – M – K unexplained global warming episodes in the past. If U is small (say 1% of the total number T), then we need to somehow explain the current warming, because our models have generally been adequate in the past. If U is large (say 50% of the total), then our failure to explain the current warming doesn’t mean much since our models repeatedly failed to explain past climate change episodes.

“So, if there have been other warming episodes in the past, is there some evidence to suggest that the current warming cannot be a repetition of any of them” – Eugene.

There is no such evidence. The enhanced Greenhouse Effect, explains a whole lot of observations and measurements. Any alternative hypothesis would have to explain those too, and also why adding an extra 37% odd CO2 to the Earth’s atmosphere would not cause additional warming.

It’s rather revealing that after nearly two centuries of research that no other valid alternative has been offered up.

“It’s rather revealing that after nearly two centuries of research that no other valid alternative has been offered up. ”

That’s why I was interested in statistics, such as the fraction of all unexplained episodes of climate change. If a lot of them are unexplained too, then maybe the fact that we cannot explain the current one without anthropogenic causes is not so revealing after all.

Well, you realize that as we go further back in time, our knowledge of what the climate was doing gets fuzzier – for anything older than 2 million years or so we are using the fraction of Oxygen 18 in fossils (to estimate the temperature) from sediment layers (whose age is estimated from other isotopes and other means). For ages less than that, we can use ice cores. Same O18 method for the temperature, I believe, but the cores can be dated quite accurately by counting layers just like tree rings. For the last one or two thousand years we have many proxies.

But here’s the most important thing to realize. Virtually all of our explanations for past climate events involve CO2 in some way. Take all the interglacial periods of the current ice age. There have been six over the past 500ky http://en.wikipedia.org/wiki/Interglacial including the current interglacial. We understand the start of these periods as caused by orbital cycles, but we can only understand the magnitude of these periods by including ice-albedo feedback, CO2 outgassing feedback, and water vapor feedback. Then, we understand the slow fall back into glacial periods as driven by CO2 scrubbing (rock weathering) CO2 sequestration by biological processes, water vapor feedback again, and ice-albedo feedback again. CO2 and H2O are essential to our understanding of natural climate change.

Other more distant climate events (see http://en.wikipedia.org/wiki/Ice_age#Major_ice_ages and note the discussion of “snowball earth”) are also understood in terms of various forcings (orbital, solar, tectonic, etc.) plus CO2 (plus H2O). For example, the only good explanation for getting out of the “snowball earth” scenario is CO2 and methane emitted from volcanoes.

So, it’s not a matter of what fraction of past climate events we think we understand. It’s that CO2 is important in understanding any of them. So finally, in a situation where humans alter the amount of CO2 in the atmosphere at a *rate* much higher than any natural process, we expect the temperature to follow suit, as the “Hockey Stick” suggests is starting to happen.

I may have given short shrift to proxy methods I’m not familiar with. There are other isotopes (e.g. C13) and other things to sample (e.g. pollen). It’s not my field!

The PETM ( http://en.wikipedia.org/wiki/PETM ) is one event where CO2 is probably not an important factor. As you can see from the article, it’s suspected that the existing slow warming (where CO2 would be involved) triggered a relatively sudden release of CH4 (methane) from ocean clathrates.

To clarify, I’m not disputing the role of CO2. I understand it’s a greenhouse gas, which means it prevents heat from escaping, which means that if you pump more of it into the atmosphere, *keeping all other things intact*, it will become hotter. Which is why it figures prominently in the past warming episodes.

What makes it a bit more difficult is that the system is complicated and if you do something, it’s not correct to assume all other things will stay intact. So I’m trying to understand how well can we predict this system. If we can predict this system fairly well, then the fact that today’s warming can only be explained anthropogenically is worth noticing. If we can predict this system only very poorly, then the fact that we can explain today’s warming anthropogenically is by itself not very convincing.

So now, how well can we predict this system?
* Take Milankovich cycles. As you say, there were six of them in the past 500ky. You also say that for this period we have relatively accurate measurements from ice cores (not as accurate as measurements of current concentrations, but still fairly accurate). Yet, it appears that the current models cannot predict these cycles accurately. Once you tell the model that a cycle begins, it will take into account temperature and CO2 and other things and will calculate the final magnitude of warming. This is a good (given how complex the problem is, even excellent) result. But the fact is, despite “centuries of research” and having relatively good data, we are not sure why the cycle begins in the first place. So the model must miss something pretty significant.
* Take PETM. (It was longer ago than 500ky, but ignore it for a moment.) The model appears adequate there, so a win for the model.
* Take some event a long long time ago which is not adequately explained by the model. You claim we can’t count it against the model since we don’t have good enough data to supply to the model. That’s fine, just note that we can’t count it in favor of the model either.

Now, the original argument in the OP is: “Even though climate has changed many times in the past, this time is different since we can’t find any natural cause. Thus, it must be anthropogenic.” Well, this doesn’t sound particularly convincing, given that of the 7 events I listed above, we have failed to find a natural cause for 6.
Of course, the list I gave is very very incomplete, which is why I asked for additional statistics.

Two more points:
“So, it’s not a matter of what fraction of past climate events we think we understand.”
– I would disagree. If our model is only good enough to understand 10% of all data, how do we know that its current predictions are right? After all, in 90% of the cases in the past they seemed right and turned out to be wrong.

“It’s that CO2 is important in understanding any of them.”
– This is not a very precise statement. I think elsewhere on this site it says that over a large time period CO2 is not correlated with temperature, because, of course, there are many other factors affecting temperature. So I agree with the weak statement that “it’s important to model CO2″ — sure, it is important. But you (and Dappledwater above) seem to imply a stronger statement — that “the current increase in CO2 must lead to warming, therefore any alternative theory must explain why this warming doesn’t happen”. As I understand, this stronger statement has no empirical support. Basically, sometimes warming started with low CO2 and sometimes there was no warming with high CO2, etc. So we can’t say something universal, like “every time CO2 is high, warming begins”. Instead, we must examine each specific instance and the exact conditions and then use the model to conclude whether or not the warming should start given current CO2 levels and other conditions. And in particular, you take the current climate model and it says that in today’s specific conditions, a level of CO2 that high must cause warming. Of course, this conclusion is only as credible as the model itself. Which leads back to my question: how many times in the past have the model’s conclusions been adequate?

Some research indicates otherwise, atmospheric levels of CO2 may have reached 1000-2000ppm as a result of the huge spike in volcanic activity as the Indian continental plate crashed into the Eurasian plate.

“it was a catastrophic event for the biosphere” – Do you think the biospehere is conscious of (or cares) what happens to it?

I don’t deny pollution in any way.

However, I am yet to be convinced that pollution affects climate as much as solar flares, the drift of the moon, changes in earth axis and polarity, forces of the earth’s crust (which inflicts a 9.0 earthquake on Japan … but isn’t even like having a pimple on the earth’s surface) and a myriad of other events constantly in change and effecting climate on earth.

The fact earth can currently support human life is in itself so statistically remote that we should rejoice for the opportunity and not be spreading fear and guilt on people for having the joy of being alive.

“…spreading fear and guilt on people for having the joy of being alive.”

So we’ve been invited to a party at the local pub. We enjoy ourselves. All well and good.

“Enjoying ourselves” at the local pub does not include throwing food at the walls and trashing the joint. We would wear out our own welcome and other people whose invitations arrived later couldn’t enjoy it at all.

Same thing goes for our unbelievable good luck in having such a congenial biosphere. Enjoyment of this incredible benefit does not give us any right to wreck it.

waguy is pretty clearly your average drive-by troll, but let me try three possible approaches in case he is not impervious (“pervious”??) to a rational dialogue:

Approach 1, point out internal inconsistencies in his argument: “I don’t deny pollution in any way” you say. Why are you spreading fear and guilt on people for having the joy of being alive?

Approach 2, engage him on his own terms: You object to the term catastrophe – “Do you think the biospehere is conscious of (or cares) what happens to it?” but there are many interpretations of catastrophe that do not depend on some afflicted consciousness to care or not care.

Approach 3, take him at his word: He says facts would convince him.
Fact 1: CO2 is the major long-lived GHG in the earth’s atmosphere, without which the average surface temperature would be about -18oC.
Fact 2: since the start of the industrial revolution fossil fuel emissions have caused an increase of some 40% in the level of atmospheric CO2.
Fact 3: this CO2 rise is greater and more rapid than any other discernible in the last several 10’s of millions of years.
Fact 4: the earth is undergoing a rapid warming event in line with the expectations of climate theory.

If those facts do not persuade you to reconsider your indifference, please be more specific as to what you need to learn.

Well it’s not far off it, Vernon. Without CO2, the =major= long-lived GHG, there certainly wouldn’t be more than a smidgin of H20 vapour. Short-lived water vapour being the biggest influence on temperatures generally.

With these 2 out of the picture, there’s not a lot of biological or other activity promoting release or persistence of the minor GHGs.

Without CO2, there might not be a nil greenhouse effect, but it would certainly be a lot less than 18C. Certainly not enough to create a Holocene with agriculture or any recognisable human civilisation.

I have to give that one to Vernon, I wrote too quickly. It would be more defensible to say that without CO2, and then because of the feedback, without H2O, we would be left with something like 5% of the current GHE. So not -18oC, but around -16.3oC, though it is not very straightforward. (refs here.)

Looking at the various papers, it appears you are referring to Gavin’s paper and his finding was that without CO2 the greenhouse effect would be 80 percent less that with CO2. Where did you find a reference for 95 percent less?

For those interested, this is a link to Gavin’s recent paper discussing this issue. A quick reading of the abstract did seem to support my 95% number, but as I said it is not really a straightforward question. Looking further into the paper I am getting the imression that removing all H2O (including clouds) and CO2 from the atmosphere leaves behind more like 9% of the current (or pre-industrial?) GHE. With no other GHG in the atmosphere, CO2, H2O, and clouds would provide 95% of the current effect. (If I am reading Table 1 correctly.)

Vernon, where do you get 80% from?

(Please note, that this does not substantially alter the argument in comment #40, where this refining the numbers exercise began)

I have a few problems with some of the discussion that has been going on with regard to this issue. There appears to be a belief – and I may be reading this wrong and if I am I apologise – that the total greenhouse effect is a sum of the various parts, with water vapour, CO2 and other gases making up the constituent parts, and that we can ‘add up’ the contribution of each gas to get an understanding of the role they play. However, this could not be more wrong.

For a start, the absorption spectrum of CO2 and water vapour overlap, so CO2 absorption at the surface (below the clouds) is significantly different to what it is in the upper atmosphere. Then there is the important issue of feedback. There will be more water vapour in warmer air, and if you decrease CO2 content, the amount of water vapour will decline. Indeed, if you were to remove ALL CO2 from the atmosphere, then there will be virtually NO water vapour (or life for that matter).

This just appears to be another example of the denier community’s lack of understanding of the basic scientific principles underpinning AGW. I know the following link is not to a peer-reviewed journal, but it gives a good overview of the history and principles behind the greenhouse theory.